text: In this boundary condition, it is assumed that on the two sides of the boundary, same physical processes exist. All the variables have same value and gradients at the same distance from the boundary. It acts as a mirror that reflects all the flow distribution to the other side.
  The conditions at symmetric boundary are no flow across boundary and no scalar flux across boundary.
  A good example is of a pipe flow with a symmetric obstacle in the flow. The obstacle divides the upper flow and lower flow as mirrored flow.